The present invention relates to a nondestructive inspecting apparatus with a SQUID (Superconducting Quantum Interference Device) and more particularly to the nondestructive inspecting apparatus with a SQUID which is preferable to sensing even a small defect of a sample with high accuracy.
Nuclear Power Plants, in general, are required to nondestructively check structures such as pipes, a pressure vessel or components such as pumps for defects like cracks or the like in the in-service inspection and, if a defect like a crack is found out, to exchange the defective structure or component before it is made more defective. As an apparatus for doing nondestructive inspection, there has been proposed an apparatus with a SQUID in addition to a ultrasonic testing device. The inspecting apparatus with a SQUID has been disclosed in J-P-A-2-78983.
FIG. 8 is a diagram showing a conventional nondestructive inspecting apparatus with a SQUID. In FIG. 8, the nondestructive inspecting apparatus for inspecting a sample 1 for cracks provides a cryostat 2 in which the SQUID itself is held. This cryostat 2 has a flexible pipe 2a connected thereto. The flexible pipe 2a provides a pick-up coil 3 at its tip 2b. The cooling medium like liquid helium held in the cryostat 2, (the pipe 2a and its tip 2b) keeps the pick-up coil 3 in a superconducting state. An exciting magnet (electromagnet) 4 is fitted around the tip 2b in a manner that the exciting magnet 4 may constantly keep an exciting state. The exciting magnet 4 is connected to an exciting control unit 5. The exciting magnet 4 and the pick-up coil 3 (held in the tip 2b) compose a sensor head. This sensor head is moved for scanning on the sample under the control of a driving unit 6.
FIG. 9 is a view showing the sensor head. When the exciting magnet 4 is dc-excited, a dc magnetic field m passes through the sample 1. In this state, the sensor head is moved toward an outlined arrow. During the scan, the dc field m crosses a crack, so that the field may change. The change of the field is sensed by the pick-up coil 3 and then is converted to an electric signal through the effect of the SQUID (not shown).
U.S. Patent application Ser. No. 07/757,585 filed 11 Sep., 1991 now U.S. Pat. No. 5,331,278, which is copending with the present application and has some common inventors, relates to the technical background of the invention of the present application.